drivers/mtd/spr_smi.c: Fix build warning

Fix:
spr_smi.c: In function 'smi_write':
spr_smi.c:325:15: warning: variable 'WM' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Vipin Kumar <vipin.kumar@st.com>