spi: Make 'default_spi_write_aai' the default unless defined

Drop the explicit need to specify the default 'default_spi_write_aai'
callback function pointer in the spi_master struct.
This is a reasonable default for every other driver in the tree
with only a few exceptions.

This simplifies the code and driver development.

Change-Id: I7f14aaea0edcf0c08cea0e9cd27d58152707fb2a
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
---
M bitbang_spi.c
M buspirate_spi.c
M ch341a_spi.c
M digilent_spi.c
M dummyflasher.c
M ft2232_spi.c
M ichspi.c
M jlink_spi.c
M linux_spi.c
M mediatek_i2c_spi.c
M mstarddc_spi.c
M ni845x_spi.c
M pickit2_spi.c
M raiden_debug_spi.c
M sb600spi.c
M serprog.c
M spi.c
M stlinkv3_spi.c
M usbblaster_spi.c
19 files changed, 21 insertions(+), 24 deletions(-)