spi: sirf: avoid uninitialized-use warning

spi: sirf: avoid uninitialized-use warning

24778be20 "spi: convert drivers to use bits_per_word_mask"
removed the "default" statement in the spi_sirfsoc_setup_transfer
switch, causing a new warning:

drivers/spi/spi-sirf.c: In function 'spi_sirfsoc_setup_transfer':
arch/arm/include/asm/io.h:90:2: warning: 'rxfifo_ctrl' may be used uninitialized in this function [-Wmaybe-uninitialized]
  asm volatile("str %1, %0"
  ^
drivers/spi/spi-sirf.c:364:19: note: 'rxfifo_ctrl' was declared here
  u32 txfifo_ctrl, rxfifo_ctrl;
                   ^

The compiler has correctly identified that this case may happen,
but since we know that things are horribly broken if bits_per_word
is ever something other than the values we tested, calling BUG()
is an appropriate action and tells the compiler that execution
will not continue afterwards.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@linaro.org>