spi: pic32: fix spelling mistakes on macro names

Trivial fix to spelling mistakes macros; fix EMPTY spellings:
RX_FIFO_EMTPY -> RX_FIFO_EMPTY
TX_FIFO_EMTPY -> TX_FIFO_EMPTY

Note that there are no other occurrances of these macros in the
source.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>