mm-modem-helpers: add low_nybble_first argument to mm_bcd_to_string

All BCD-encoded strings used by MM currently have the low nybble of each byte come before the high nybble, but some strings (such as the EID string returned by QMI Get Slot Status) are meant to be read in order with the high nybble before the low one. As such, extend mm_bcd_to_string to decode both.