mISDN: mark symbols static where possible

We get a few warnings when building kernel with W=1:
drivers/isdn/hardware/mISDN/hfcmulti.c:568:1: warning: no previous declaration for 'enablepcibridge' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:574:1: warning: no previous declaration for 'disablepcibridge' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:580:1: warning: no previous declaration for 'readpcibridge' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:608:1: warning: no previous declaration for 'writepcibridge' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:638:1: warning: no previous declaration for 'cpld_set_reg' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:645:1: warning: no previous declaration for 'cpld_write_reg' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:657:1: warning: no previous declaration for 'cpld_read_reg' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:674:1: warning: no previous declaration for 'vpm_write_address' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:681:1: warning: no previous declaration for 'vpm_read_address' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:695:1: warning: no previous declaration for 'vpm_in' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:716:1: warning: no previous declaration for 'vpm_out' [-Wmissing-declarations]
drivers/isdn/hardware/mISDN/hfcmulti.c:1028:1: warning: no previous declaration for 'plxsd_checksync' [-Wmissing-declarations]
....

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
so this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>