eeprom: at24: code shrink

A regmap_config struct is pretty big and declaring two of them
statically just to tweak the reg_bits value adds unnecessary bloat.

Declare the regmap config locally in at24_probe() instead.

Bloat-o-meter output for ARM:

add/remove: 0/2 grow/shrink: 1/0 up/down: 4/-272 (-268)
Function                                     old     new   delta
at24_probe                                  1560    1564      +4
regmap_config_8                              136       -    -136
regmap_config_16                             136       -    -136
Total: Before=7012, After=6744, chg -3.82%

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>