serial: pic32: simplify pic32_sport::enable_tx_irq handling

Make it a bool, so use true+false. And remove the wrap-around macro --
i.e. access the member directly.

It makes the code more obvious.

BTW the macro did not have 'sport' in parentheses, so it was potentially
problematic wrt expansion.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220503063122.20957-6-jslaby@suse.cz


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