greybus: don't key on an enumerated constant

The symbol PWMF_ENABLED is defined via an enum, which is not defined
at the time the preprocessor passes through "kernel_ver.h".  So we
can't use it in an #if statement expression.

Use the Linux kernel version instead.

Change-Id: Id427224b1dfecfd886fcdae89c4bcf711b616439
Signed-off-by: Alex Elder <elder@linaro.org>