rmnet_perf: Spinlock fix for module param

Previously rmnet_perf_opt_mode was spinlocking without disabling
softirqs. This caused a potential deadlock if it was preempted on
the core rmnet would try to spinlock from.

This change adds softirq disable before attempting to lock preventing
this scenario

Change-Id: Ic56820dd9462ea81f11b22abb1314dca0ec9fd94
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>