Revert "Bug 876369 - Part 5: Use amodem_nvram_set to set and save nvram ...

...config"

`amodem_nvram_set()` depends on `modem->nvram_config` being set
previously, but it's actually set by the return value of
`amodem_load_nvram`.  So `modem->nvram_config` is not yet initialized
here and traps emulator with segmentation fault.

Commit aece119 was meant to share common code for saving nvram
configs. It only replaces lines in two places, inclusive of
`amodem_load_nvram`.  Since we shouldn't call `amodem_nvram_set()`
here, leaving sharing common code a meaningless thing, we should
revert it instead.

This reverts commit aece119472ba3509202de0d2b25167b1237cd0c2.