sfxge(4): fix align to power of 2 when align has smaller type

Substitute driver-defined P2ALIGN() with EFX_P2ALIGN() defined in
libefx.

Cast value and alignment to one specified type to guarantee result
correctness.

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/D21075