ar5523: make buffer size variable unsigned

A negative buffer size doesn't make sense and it breaks this check in
ar5523_get_max_rxsz():

	if (!ar->rxbufsz || ar->rxbufsz > AR5523_SANE_RXBUFSZ) { ...

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>