qcom: pil-femto-modem: Don't use the default unmap_fw_mem implementation

Since pil-femto-modem is using its own version of the
map_fw_mem call, don't use the default unmap_fw_mem, as
the latter may use a memory de-allocation API that is
incompatible with the allocation API used in pil-femto-modem's
map_fw_mem.

Change-Id: I7afec565b9fa76491881ea6f1d6b84f920ad8d4a
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>