drivers: spi: sam0: Handle 32-bit length extension

The sam0 SPI driver does not ensure that it clears the 32-bit extension
option during init. The 32-bit extension option, which comprises of a field
in the CTRLC register and the LENGTH register enables better bus
utilization by allowing 32-bit writes to the SPI DATA register
(as opposed to the usual 8-bit writes). The driver breaks down if this
option is enabled by causing each intended byte of output to become
four bytes. We fix this by explicitly disabling the 32-bit extension
option in init.

Signed-off-by: Pete Dietl <petedietl@gmail.com>