staging: fsl-mc: Avoid section mismatch

The fsl_mc_allocator_driver_exit() function is marked __exit, but is
called by the error handling code in fsl_mc_allocator_driver_init().
This results in a section mismatch, which in turn could lead to
executing random code.

Remove the __exit annotation to fix this.

Cc: J. German Rivera <German.Rivera@freescale.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>