sfxge(4): unify power of 2 alignment check macro

Substitute driver-defined IS_P2ALIGNED() with EFX_IS_P2ALIGNED()
defined in libefx.

Add type argument and cast value and alignment to one specified type.

Reported by: Andrea Valsania <andrea.valsania at answervad.it>
Reviewed by: philip
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D21076