hamradio: baycom_epp: Do not use x86-specific rdtsc()

Use get_cycles() which is provided by pretty much every arch.

The UML build works too because get_cycles() is a simple "return 0;"
because the rdtsc() is optimized away there.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: David S. Miller <davem@davemloft.net>