regmap: correct NULL pointer access

dev_get_regmap can return NULL and hence map can be NULL.
So check if map is a valid pointer before accessing.

Change-Id: Ied358e0fed551bf0b96e6cd2e0b44ebde990621f
Signed-off-by: Yeleswarapu Nagaradhesh <nagaradh@codeaurora.org>