mwifiex: Use accessors routines for unaligned values

Synopsys' ARCompact architecture does not support loading from or
storing values to unaligned memory locations. We saw a series of
misaligned access exceptions on ARC. To work around this issue, we bulk
replaced le16_to_cpu and le32_to_cpu with get_unaligned_le16 and
get_unaligned_le32, respectively. We also added le16_unaligned_add_cpu
which is similar to le16_add_cpu but works with unaligned values.

Signed-off-by: Daniel Mentz <danielmentz@google.com>
Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>