drivers: spi: nrfx: Restore recently changed spi_context function

Function spi_context_longest_current_buf() has been introduced in
commit ddef35c1 for the purpose of
getting the longest possible (potentially partial) SPI transfer
for which all currently active directions have a continuous buffer.
Such transfer can be done with taking advantage of a DMA that cannot
use scattered buffers (and this is the case for nRF SPI drivers with
which this function has been introduced).
Unfortunately, because of its inadequate name, later on this function
has been incorrectly used in other SPI drivers for getting the longer
of TX/RX buffers. And commit afc480f1
recently "fixed" the implementation of this function, assumably to
adjust it to those incorrect uses, but this way it has also broken
the nRF SPI drivers.
Instead of restoring the original implementation of the function in
question, this commit adds a new one with functionality equivalent
to that original but with a hopefully less misleading name, and this
function is used in the nRF SPI drivers.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>