i2c: eg20t: pass on return value in i2c_xfer

smatch says:
drivers/i2c/busses/i2c-eg20t.c:702 pch_i2c_xfer() info: why not propagate 'ret' from mutex_lock_interruptible() instead of -512?

which is especially true since for -ENORESTARTSYS it is said in
linux/errno.h "signal_pending() MUST be set" which is not done here. So
just pass on the return value we got.

Signed-off-by: Wolfram Sang <wolfram@the-dreams.de>
Cc: Tomoya MORINAGA <tomoya.rohm@gmail.com>