tree/: Drop default_spi_probe_opcode for NULL case

A NULL func pointer is necessary and sufficient for the
condition `NULL func pointer => true' as to not need this
boilerplate as it implies default behaviour of a supported
opcode within the `check_block_eraser()` match supported loop.

Ran;
```
$  find . -name '*.[c,h]' -exec sed -i '/.probe_opcode	= default_spi_probe_opcode,/d' '{}' \;
```

Change-Id: Id502c5d2596ad1db52faf05723083620e4c52c12
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 dirtyjtag_spi.c
M flashrom.c
M ft2232_spi.c
M include/chipdrivers.h
M include/programmer.h
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 spi25_statusreg.c
M stlinkv3_spi.c
M usbblaster_spi.c
M wbsio_spi.c
27 files changed, 30 insertions(+), 32 deletions(-)