Build fails on 4.19

[vleschuk]

  CC [M]  /var/lib/dkms/crystalhd/3.10.0/build/crystalhd/driver/linux/crystalhd_hw.o
/var/lib/dkms/crystalhd/3.10.0/build/crystalhd/driver/linux/crystalhd_hw.c: In function ‘crystalhd_rx_pkt_done’:
/var/lib/dkms/crystalhd/3.10.0/build/crystalhd/driver/linux/crystalhd_hw.c:658:6: error: implicit declaration of function ‘rdtscll’; did you mean ‘rdtsc’? [-Werror=implicit-function-declaration]
      rdtscll(currTick);
      ^~~~~~~
      rdtsc
/var/lib/dkms/crystalhd/3.10.0/build/crystalhd/driver/linux/crystalhd_hw.c:658:6: error: ‘currTick’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
      rdtscll(currTick);
      ^~~~~~~~~~~~~~~~~

My kernel:

$ uname -a

Linux thinkpad 4.19.0-5-amd64 #1 SMP Debian 4.19.37-3 (2019-05-15) x86_64 GNU/Linux

Fix from #23 works. Could you please merge it?