staging: brcm80211: use min_t() instead of min()

Also eliminate a cast to int that could cause issues with very large values
of di->rxbufsize (as suggested by Dan Carpenter).

Signed-off-by: Henry Ptasinski <henryp@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>