Blackfin: bfin_gpio: Use proper mask for comparing pfunc

For BF537_FAMILY, when offset != 1, the mask is 1.
Thus add proper mask for comparing pfunc with function.

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

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>