docs: Fix bitfield handling in kernel-doc

docs: Fix bitfield handling in kernel-doc

kernel-doc doesn't handle bitfields that are specified with symbolic
name, e.g. u32 cs_index_mask : SPI_CS_CNT_MAX

This results in the following warnings when running `make htmldocs`:

include/linux/spi/spi.h:246: warning: Function parameter or struct member 'cs_index_mask:SPI_CS_CNT_MAX' not described in 'spi_device'
include/linux/spi/spi.h:246: warning: Excess struct member 'cs_index_mask' description in 'spi_device'

Update the regexp for bitfields to accept all word chars, not just
digits.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240326173825.99190-1-donald.hunter@gmail.com