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

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Cc: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>