sh: 16-bit get_unaligned() sh4a fix

This patch fixes the 16-bit case of the sh4a specific
unaligned access implementation. Without this patch
the 16-bit version of sh4a get_unaligned() results in
a 32-bit read which may read more data than intended
and/or cross page boundaries.

Unbreaks mtd NOR write handling on Migo-R.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>