ANSI683: Fix mobile country code base

Fixes error:
Field 'Mobile country code (MCC_T)' (ansi_683.mcc_t) is an integral value
(FT_UINT16) but is being displayed as BASE_NONE

Change-Id: I2d87075b0c982bcaf0e855653d49eb891e83b70d
Reviewed-on: https://code.wireshark.org/review/4536
Reviewed-by: Evan Huus <eapache@gmail.com>