TPA613A2 driver should be updated to use `fp_conversions.h`

[tvannoy]

Since there were some bug fixes in the fixed-point conversion functions (#3), we should update tpa613a2.c to use fp_conversions.h, as the bug is probably in tpa613a2.c as well.

str_to_fp has diverged from it's implementation as set_fixed_num in tpa613a2.c, however, so this isn't a 100% trivial find/replace operation.