staging: rtl8192u: fix a dubious looking mask before a shift

Currently the masking of ret with 0xff and followed by a right shift
of 8 bits always leaves a zero result.  It appears the mask of 0xff
is incorrect and should be 0xff00, but I don't have the hardware to
test this. Fix this to mask the upper 8 bits before shifting.

[ Not tested ]

Addresses-Coverity: ("Operands don't affect result")
Fixes: 8fc8598e

 ("Staging: Added Realtek rtl8192u driver to staging")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200716154720.1710252-1-colin.king@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>