ALSA: ua101: replace le16_to_cpu() with usb_endpoint_maxp()

Commit 939f325f ("usb: add usb_endpoint_maxp() macro") and commit
29cc8897 ("USB: use usb_endpoint_maxp() instead of le16_to_cpu()")
introduced a new helper macro.  This trivial patch convert remaining
users found in ua101 driver.

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>