samples/nfc: Remove x86 asm code

This commit replaces the x86 asm line:

__asm__ __volatile__ ("bswap %0" : "=r" (x) : "0" (x));

by the sys_cpu_to_be32 macro.

By removing the x86 asm instruction, the nfc_hello sample app
can run on other platforms.

Jira: ZEP-1348

Change-Id: I440e96cd06e70a88552d179a2288c9918e6ca0b0
Signed-off-by: Flavio Santes <flavio.santes@intel.com>