regmap: mmio: Fix regmap_mmio_write for uneven counts

Commit 93258040
"regmap: mmio: Add support for 1/2/8 bytes wide register address."
broke regmap_mmio_write for uneven counts, for example 32-bit register
addresses with no padding and 8-byte values (count = 5).
Fix this by allowing all counts large enough to include some value.
This check was BUG_ON(count < 4) before the last change.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>