serial: qcom: use check for empty instead of pending

The code wants to know if the circ buffer is empty, so use the proper
macro.

No functional change intended, just saner function name used for that
use case.

Cc: Andy Gross <agross@kernel.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220421101708.5640-6-jslaby@suse.cz


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