net: zd1211rw: Remove ZD_ASSERT(in_interrupt())

in_interrupt() is ill defined and does not provide what the name
suggests. The usage especially in driver code is deprecated and
a tree wide effort to clean up and consolidate the (ab)usage of
in_interrupt() and related checks is happening.

handle_regs_int() is always invoked as part of URB callback which is either
invoked from hard or soft interrupt context.

Remove the magic assertion.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>