Staging: rts5139: Fix incorrect type in assignment in rts51x_fop.c

This patch fixes following sparse warning in rts51x_fop.c
Warning: incorrect type in assignment

Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>