zephyr samples: fix wolfssl_benchmark RSA.

[philljj]

Description

Fix RSA failure in zephyr wolfssl_benchmark example.

Testing

Before:

$west build -p auto -b qemu_x86 modules/crypto/wolfssl/zephyr/samples/wolfssl_benchmark
...
$west build -t run
...
[00:00:38.700,000] <err> os: CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE is 0
RSA     2048   public         0 ops took 0.010 sec, avg 0.000 ms, 0.000 ops/sec
Benchmark RSA public 2048 failed: -112
DH      2048  key gen         1 ops took 1.680 sec, avg 1680.000 ms, 0.595 ops/sec
DH      2048    agree         2 ops took 6.610 sec, avg 3305.000 ms, 0.303 ops/sec
ECC   [      SECP256R1]   256  key gen         2 ops took 3.420 sec, avg 1710.000 ms, 0.585 ops/sec
ECDHE [      SECP256R1]   256    agree         2 ops took 3.410 sec, avg 1705.000 ms, 0.587 ops/sec
ECDSA [      SECP256R1]   256     sign         2 ops took 3.480 sec, avg 1740.000 ms, 0.575 ops/sec
ECDSA [      SECP256R1]   256   verify         2 ops took 6.600 sec, avg 3300.000 ms, 0.303 ops/sec
Benchmark complete

After:

$west build -p auto -b qemu_x86 modules/crypto/wolfssl/zephyr/samples/wolfssl_benchmark
...
$west build -t run
...
RSA     2048   public         8 ops took 1.190 sec, avg 148.750 ms, 6.723 ops/sec
RSA     2048  private         2 ops took 17.650 sec, avg 8825.000 ms, 0.113 ops/sec
DH      2048  key gen         1 ops took 1.680 sec, avg 1680.000 ms, 0.595 ops/sec
DH      2048    agree         2 ops took 6.610 sec, avg 3305.000 ms, 0.303 ops/sec
ECC   [      SECP256R1]   256  key gen         2 ops took 3.420 sec, avg 1710.000 ms, 0.585 ops/sec
ECDHE [      SECP256R1]   256    agree         2 ops took 3.410 sec, avg 1705.000 ms, 0.587 ops/sec
ECDSA [      SECP256R1]   256     sign         2 ops took 3.470 sec, avg 1735.000 ms, 0.576 ops/sec
ECDSA [      SECP256R1]   256   verify         2 ops took 6.600 sec, avg 3300.000 ms, 0.303 ops/sec
Benchmark complete

[philljj]

Retest this please.