net: stmmac: Use mutex instead of spinlock

fix_speed, clk_set_rate functions need to sleep under a lock.
So there is no need for spinlock. Replaced spinlock with mutex.

Change-Id: Icf0c7d15585bbd2ce370df0c2ff32e1fc5a5d384
Signed-off-by: Sneh Shah <snehshah@codeaurora.org>