Serial 8250: handle saving the clear-on-read bits from the LSR and MSR

Reading the LSR clears the break, parity, frame error, and overrun bits in
the 8250 chip, but these are not being saved in all places that read the
LSR.  Same goes for the MSR delta bits.  Save the LSR bits off whenever the
lsr is read so they can be handled later in the receive routine.  Save the
MSR bits to be handled in the modem status routine.

Also, clear the stored bits and clear the interrupt registers before
enabling interrupts, to avoid handling old values of the stored bits in the
interrupt routines.

[[email protected]: clean up pre-existing code]
Signed-off-by: Corey Minyard <[email protected]>
Cc: Russell King <[email protected]>
Cc: Yinghai Lu <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Acked-by: Alan Cox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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