drivers: spi_ll_stm32: fix COND_CODE_1 usage

The consequent and alternate expressions for COND_CODE_1 must be
enclosed in parentheses, like this:

COND_CODE_1(PREDICATE, (consequent), (alternate))

The parens are missing in exactly one place in the tree. Fix it.

Reported-by: Peter Bigot <peter.bigot@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>