auxdisplay: img-ascii-lcd: Silence 2 uninitialized warnings

The warnings are:

  drivers/auxdisplay/img-ascii-lcd.c: warning: 'err' may be used
  uninitialized in this function [-Wuninitialized]

At lines 109 and 207. Reported by Geert using the build service
several times, e.g.:

  https://lkml.org/lkml/2018/2/19/303



They are two false positives, since num_chars > 0 in the three present
configurations (boston, malta, sead3). Initialize to 0 in order to
silence the warning.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Paul Burton <paul.burton@mips.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>