drivers/char/mxser.c: fix compilation warning in mxser.c
Both mxser_disable_must_enchance_mode() and mxser_get_must_hardware_id()
called from function CheckIsMoxaMust(), when CONFIG_PCI=y. So mark both
the functions under CONFIG_PCI.
We were warned by the following warning.
drivers/char/mxser.c:306: warning: `mxser_disable_must_enchance_mode' defined but not used
drivers/char/mxser.c:391: warning: `mxser_get_must_hardware_id'
defined but not used