iio: accel: sca3000_core: avoid potentially uninitialized variable

The newly added __sca3000_get_base_freq function handles all valid
modes of the SCA3000_REG_ADDR_MODE register, but gcc notices
that any other value (i.e. 0x00) causes the base_freq variable to
not get initialized:

drivers/staging/iio/accel/sca3000_core.c: In function 'sca3000_write_raw':
drivers/staging/iio/accel/sca3000_core.c:527:23: error: 'base_freq' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This adds explicit error handling for unexpected register values,
to ensure this cannot happen.

Fixes: e0f3fc9b ("iio: accel: sca3000_core: implemented IIO_CHAN_INFO_SAMP_FREQ")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Ico Doornekamp <ico@pruts.nl>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>