mailbox: bcm-flexrm-mailbox: fix spelling mistake "toogle" -> "toggle"

Trivial fix to spelling mistake in function name flexrm_flip_header_toogle,
rename it to flexrm_flip_header_toggle.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>