regmap: mmio: Fix value endianness selection

Currently when selecting value endianness we check the register
endiannes, not the value endianness.

Reported-by: Alexander Stein <alexander.stein@systec-electronic.com>
Tested-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Mark Brown <broonie@kernel.org>