fsi: Move fsi_slave structure definition to header

Some FSI drivers may have need of the slave definition, so
move it to a header file. Also use one macro for obtaining a
pointer to the fsi_master structure.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20230612195657.245125-2-eajames@linux.ibm.com
Signed-off-by: Joel Stanley <joel@jms.id.au>