staging: greybus/timesync: avoid divide by zero on X86 Qemu

A system configured without CONFIG_CPUFREQ will return 0 for cpufreq_get().
greybus-timesync can subsequently then do a divide-by-zero as result. This
patch fixes by checking for a zero return value from cpufreq_get() and
setting to a default value of 19.2MHz.

Reported-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>