tty: serial: tegra: check function's return value

Platform_get_irq returns int. So, the check should
be (int)irq < 0 not irq < 0 which has no effect as irq
is unsigned.

Bug 1046331

Change-Id: I6599f34ef42bcd286908b4bdd029639231fb9a3d
Signed-off-by: Sri Krishna chowdary <schowdary@nvidia.com>
Reviewed-on: http://git-master/r/159553
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>