net: qualcomm: prepare frame decoding for UART driver

Unfortunately the frame format is not exactly identical between SPI
and UART. In case of SPI there is an additional HW length at the
beginning. So store the initial state to make the decoding state machine
more flexible and easy to extend for UART support.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: David S. Miller <davem@davemloft.net>