mxser: simplify mxser_interrupt and drop mxser_board::vector_mask

mxser_board::vector_mask is just a bitfield with bits set for all
available ports.  We can obtain this value simply by
"BIT(brd->info->nports) - 1" directly in the ISR. So remove vector_mask
and simplify the code a bit.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-15-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>