p54: fix powerpc gcc warnings

My commit "p54: parse output power table" introduced
the following compiler warnings for powerpc-allmodconfig

eeprom.c: In function 'p54_get_maxpower':
eeprom.c:291 warning: comparison of distinct pointer types lacks a cast
eeporm.c:292 warning: comparison of distinct pointer types lacks a cast
eeprom.c:293 warning: comparison of distinct pointer types lacks a cast
eeprom.c:294 warning: comparison of distinct pointer types lacks a cast

This patch fixes those by using max_t(u16
which forces a type cast.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Tested-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>