mISDN: remove unused vpm_read_address and cpld_read_reg functions

clang with W=1 reports
drivers/isdn/hardware/mISDN/hfcmulti.c:667:1: error: unused function
  'vpm_read_address' [-Werror,-Wunused-function]
vpm_read_address(struct hfc_multi *c)
^

drivers/isdn/hardware/mISDN/hfcmulti.c:643:1: error: unused function
  'cpld_read_reg' [-Werror,-Wunused-function]
cpld_read_reg(struct hfc_multi *hc, unsigned char reg)
^

These functions are not used, so remove them.

Reported-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230323161343.2633836-1-trix@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>