spi: Make 'default_spi_send_multicommand' the default unless defined

A NULL func pointer is necessary and sufficient for the
condition `NULL func pointer => default_spi_send_multicommand' as to not
need this explicit specification of 'default'.

Therefore drop the explicit need to specify the 'default_spi_send_multicommand'
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: I6cc24bf982da3d5251d391eb397db43dd10280e8
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
---
M bitbang_spi.c
M buspirate_spi.c
M ch341a_spi.c
M dediprog.c
M digilent_spi.c
M dummyflasher.c
M it87spi.c
M jlink_spi.c
M linux_spi.c
M mediatek_i2c_spi.c
M mstarddc_spi.c
M ni845x_spi.c
M parade_lspcon.c
M pickit2_spi.c
M raiden_debug_spi.c
M realtek_mst_i2c_spi.c
M sb600spi.c
M serprog.c
M spi.c
M stlinkv3_spi.c
M usbblaster_spi.c
M wbsio_spi.c
22 files changed, 24 insertions(+), 27 deletions(-)