serial: max310x: Fix tx_empty() callback

serial: max310x: Fix tx_empty() callback

BugLink: https://bugs.launchpad.net/bugs/1855787
[ Upstream commit a8da3c7873ea57acb8f9cea58c0af477522965aa ]

Function max310x_tx_empty() accesses the IRQSTS register, which is
cleared by IC when reading, so if there is an interrupt status, we
will lose it. This patch implement the transmitter check only by
the current FIFO level.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>