i40iw: Redefine i40iw_mmiowb() to do nothing

mmiowb() is now implicit in spin_unlock(), so there's no reason to call
it from driver code. Redefine i40iw_mmiowb() to do nothing instead.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>