iio: light: isl29501: Simplify code to kill compiler warning

With gcc 4.1.2:

    drivers/iio/proximity/isl29501.c: In function ‘isl29501_register_write’:
    drivers/iio/proximity/isl29501.c:235: warning: ‘msb’ may be used uninitialized in this function

While this is a false positive, it can easily be avoided by removing the
"msb" intermediate variable.
Remove the "lsb" intermediate variable for consistency.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>