drivers: serial: uart_sam: Fix typo in param name.

The refactor to add callback user data param in 57286afd, contained
typo just for this driver. It sneaked past the PR CI due to the fact
that issue affected just a couple of platforms, and we select just
a few of them randomly for PR CI (vs full CI).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
1 file changed