netlib_xarp: remove redundant cast

[zhhyu7]

Summary

removes redundant type casts in ARP-related network library functions. The casts from req.arp_dev (already a char array) to (FAR char *) are unnecessary and can be safely removed.

Impact

Code cleanup only, no functional changes;
Improves code readability by removing unnecessary casts.

Testing

sim:matter

• [*] Build tested
• [*] Functional testing of ARP commands
  NuttX test log

NuttShell (NSH) NuttX-12.12.0
MOTD: username=admin password=Administrator
nsh> 
nsh> arp
IP Address   Ethernet Address  Interface
nsh> help arp
arp usage:  arp [-i <ifname>] [-a <ipaddr>|-d <ipaddr>|-s <ipaddr> <hwaddr>]
nsh> arp -i eth0 -s 10.0.1.3 00:11:22:33:44:55
nsh> arp
IP Address   Ethernet Address  Interface
10.0.1.3     00:11:22:33:44:55 eth0
nsh> arp -i eth0
IP Address   Ethernet Address  Interface
10.0.1.3     00:11:22:33:44:55 eth0
nsh> arp -a 10.0.1.3
HWaddr: 00:11:22:33:44:55
nsh> arp -d 10.0.1.3
nsh> arp -a 10.0.1.3
nsh: arp: no such ARP entry: 10.0.1.3
nsh> arp -i eth0
IP Address   Ethernet Address  Interface
nsh> arp
IP Address   Ethernet Address  Interface
nsh>