Staging: rtlwifi: base: Modified the line ending with a parenthesis

This patch fixes the "Lines should not end with a '('" warning reported by
checkpatch.pl script. The line containing 'rtl_mrate_idx_to_arfr_id' function
previously ended with '(', which did not conform to the linux kernel
coding style.

Signed-off-by: Sohil Ladhani <supertakumi86@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>