serial: core: Fix runtime PM handling for pending tx

Richard reported that a serial port may end up sometimes with tx data
pending in the buffer for long periods of time.

Turns out we bail out early on any errors from pm_runtime_get(),
including -EINPROGRESS. To fix the issue, we need to ignore -EINPROGRESS
as we only care about the runtime PM usage count at this point. We check
for an active runtime PM state later on for tx.

Fixes: 84a9582fd203 ("serial: core: Start managing serial controllers to enable runtime PM")
Cc: stable <[email protected]>
Reported-by: Richard Purdie <[email protected]>
Cc: Bruce Ashfield <[email protected]>
Cc: Mikko Rapeli <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Randy MacLeod <[email protected]>
Signed-off-by: Tony Lindgren <[email protected]>
Tested-by: Richard Purdie <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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