blackfin: gpio: Use proper mask for comparing function

The function return from P_FUNCT2MUX(per) takes 2 bits, however
for BF537_FAMILY with offset != 1 the function is 1 bit.

Also has small refactor for better readability.
In portmux_setup(), it looks odd having "muxreg &= ~(3 << 1);"
while in current code we do muxreg |= (function << offset);.

Signed-off-by: Axel Lin <axel.lin@ingics.com>