rt2x00: Compiler warning unmasked by fix of BUILD_BUG_ON

A "Set" to a sign-bit in an "&" operation causes a compiler warning.
Make calculations unsigned.

[ The warning was masked by the old definition of BUILD_BUG_ON() ]

Also remove __builtin_constant_p from FIELD_CHECK since BUILD_BUG_ON
no longer permits non-const values.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
CC: Ingo Molnar <mingo@elte.hu>
CC: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>