drivers: fmc: core: load at24 before probing fmc

In cases where driver modules are not built into the kernel itself, at24
module is not loaded at the desired time, and thus, can cause error when
trying to read the eeprom.

Also, listing at24 as a soft dependency doesn't solves our problem as it
does not guarantees the time at which the module will be loaded.

Hence, load the module using request_module(), and throw a kernel warning
in syslog if it fails.
Signed-off-by: Vaibhav Gupta <vaibhav.gupta@cern.ch>