serial: qcom_geni: Fix variable naming

Commit 2aaa43c7 ("tty: serial: qcom-geni-serial: add support for
serial engine DMA") renamed rx_fifo member to rf_buf which caused a
build failure when b8caf69a ("tty: serial: qcom-geni-serial: fix
slab-out-of-bounds on RX FIFO buffer") from tty-linus was merged into
tty-next.

Fix the member variable name.

Fixes: 7a6aa989

 ("Merge 6.2-rc5 into tty-next")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230123173857.40695-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>