[SERIAL] remove unneeded code from serial_core.c

This patch fixes an issue reported by Coverity in serial/serial_core.c

Error reported: Variable "&((info)->tty)->flags" tracked as NULL was passed to a
function that dereferences it.

The later statements in the function assumes 'info->tty != NULL', so this
check is not necessary.  Probably a 'BUG_ON(info->tty == NULL)' can be added.

Signed-off-by: Jayachandran C. <c.jayachandran at gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>