mfd: sm501: Avoid implicit sign extension

mfd: sm501: Avoid implicit sign extension

Suspicious implicit sign extension where 'reg' (unsigned char) is
promoted in reg << clksrc to int, then sign-extended to unsigned
long. If reg << clksrc is greater than 0x7FFFFFFF, the upper bits
of the result will all be 1.

Signed-off-by: Lee Jones <lee.jones@linaro.org>