staging: gs_fpgpaboot: fix compiler warning with make W=1

the following compiler warning has been fixed:

drivers/staging/gs_fpgaboot/gs_fpgaboot.c: In function ‘gs_read_bitstream’:
drivers/staging/gs_fpgaboot/gs_fpgaboot.c:160:6: warning: variable ‘size’ set but not used [-Wunused-but-set-variable]
  CC      drivers/staging/gs_fpgaboot/io.o
  LD      drivers/staging/gs_fpgaboot/gs_fpga.o
  LD      drivers/staging/gs_fpgaboot/built-in.o

by removing the size variable.

Signed-off-by: Devendra Naga <devendranaga4@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>