auxdisplay: charlcd: fix x/y command parsing

The x/y command parsing has been broken since commit 12995706
("staging: panel: Fixed checkpatch warning about simple_strtoul()").

Commit b34050fa ("auxdisplay: charlcd: Fix and clean up handling of
x/y commands") fixed some problems by rewriting the parsing code,
but also broke things further by removing the check for a complete
command before attempting to parse it.  As a result, parsing is
terminated at the first x or y character.

This reinstates the check for a final semicolon.  Whereas the original
code use strchr(), this is wasteful seeing as the semicolon is always
at the end of the buffer.  Thus check this character directly instead.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>