staging/rts5208: fix incorrect shift to extract upper nybble

The mask of sns_key_info1 suggests the upper nybble is being extracted
however the following shift of 8 bits is too large and always results in
0.  Fix this by shifting only by 4 bits to correctly get the upper nybble.

Detected by CoverityScan, CID#142891 ("Operands don't affect result")

Fixes: fa590c22 ("staging: rts5208: add support for rts5208 and rts5288")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>