staging: rts5208: fix missing error check on call to rtsx_write_register

Currently the check on error return from the call to rtsx_write_register
is checking the error status from the previous call. Fix this by adding
in the missing assignment of retval.

Detected by CoverityScan, CID#709877

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