drivers: spi_context: Functions for remaining len

Add functions to get remaining length in the transfer. Refactor a bit to
avoid duplicate code for the for loop that is the same as in the total
length function.

The difference between the total length and left length function is that
the current buffer total length is counted in the former and the current
buffer remaining length is counted in the latter.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>