serial: 8250: 8250_omap: Fix RX_LVL register offset

Commit b67e830d ("serial: 8250: 8250_omap: Fix possible interrupt
storm on K3 SoCs") introduced fixup including a register read to
RX_LVL, however, we should be using word offset than byte offset
since our registers are on 4 byte boundary (port.regshift = 2) for
8250_omap.

Fixes: b67e830d ("serial: 8250: 8250_omap: Fix possible interrupt storm on K3 SoCs")
Cc: stable <stable@vger.kernel.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20210903050550.29050-1-nm@ti.com


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