mx25 clocks: Fix MXC_FEC_CLK

mxc_get_clock(MXC_FEC_CLK) should return the IPG clock, not the AHB clock.

Also, imx_get_fecclk() was correct but reimplemented the calculation of the IPG
clock, so remove the duplicated code.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>