mfd: rave-sp: Check received frame length before accepting next byte

Check received frame length _before_ accepting next byte in order to
avoid incorrectly rejecting payloads that are RAVE_SP_RX_BUFFER_SIZE
long.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Tested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>