net: cxbg4: Remove pointless in_interrupt() check

t4_sge_stop() is only ever called from task context and the in_interrupt()
check is presumably a leftover from copying t3_sge_stop().

Aside of in_interrupt() being deprecated because it's not providing what it
claims to provide, this check would paper over illegitimate callers.

The functions invoked from t4_sge_stop() contain already warnings to catch
invocations from invalid contexts.

Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>