mfd: si476x: Use get_unaligned_be16() for unaligned be16 loads

Loading be16 values from byte buffers may cause unaligned accesses, so use
get_unaligned_be16() to avoid problems on architectures that do not support
these.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>