rtl8192cu: Fix unbalanced irq enable in error path of rtl92cu_hw_init()

Fixes: a53268be

 ('rtlwifi: rtl8192cu: Fix too long disable of IRQs')
Cc: stable@vger.kernel.org
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>