mxser: move MSR read to mxser_check_modem_status()

The MSR read is currently performed on both places where
mxser_check_modem_status() is called. So move it there to avoid code
duplication.

Rename the variable to msr while we move it, to actually see what
"status" we are testing.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-5-jslaby@suse.cz


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