serial: omap: drop "inline" from IRQ handler prototype

it makes no sense to mark our IRQ handler inline
since it's passed as a function pointer when
enabling the IRQ line.

Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>