atmel: Remove open-coded and wrong strcasecmp

The kernel's string library does in fact have strcasecmp, at least
since ded220bd ("[STRING]: Move strcasecmp/strncasecmp to
lib/string.c"). Moreover, this open-coded version is in fact wrong: If
the strings only differ in their last character, a and b have already
been incremented to point to the terminating NUL bytes, so they would
wrongly be treated as equal.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>